In all internal combustion engines, IC engines, the air/fuel ratio is of utmost importance for the engine function. Usually the air/fuel ratio is referred to as the A/F-ratio, A and F signifying respectively air and fuel. In order to achieve a satisfactory combination of low fuel consumption, low exhaust emissions, good runability and high efficiency the A/F-ratio must be maintained within comparatively narrow limits.
The requirements that exhaust emissions from the IC engine to be kept low are becoming increasingly stricter. In the case of car engines these requirements have led to the use of exhaust catalysers and to the use of sensors and probes positioned in the car exhaust system in order to control the A/F-ratio.
However, for consumer products, such as power saws, lawn mowers, and similar products, this technology is difficult to use for mounting reasons and also for cost—efficiency and operational—safety reasons. For instance, in a power saw, a system with sensors and probes would result in increased size and weight as well as a drastic rise in costs and possibly also cause operational safety problems. Further the sensor or the probe often requires a reference having completely pure oxygen, which is a situation that it is practically impossible to achieve in some engines, for instance the motors of power saws.
Expected future legislation with respect to CO-emissions from small IC engines may make it difficult to use manually adjusted carburettors. Given the manufacturing tolerances that could be achieved in the case of carburettors it is impossible, with the use of fixed nozzles in the carburettor, to meet these legal requirements and at the same time guarantee the user good runability in all combinations of air-pressures and temperatures, different fuel qualities and so on.
EP 0 715 686 B1 describes a method of controlling the engine A/F-ratio without the use of an oxygen sensor (lambda probe). Initially, the A/F-ratio is changed briefly. This could be effected for instance by briefly throttling or stopping the fuel supply. In connection with the change, a number of engine revolution times are measured. The revolution times relate to engine rotational speeds chosen in such a manner that at least one revolution of the engine is unaffected by the change, preferably an engine rotational speed that is sufficiently early for the A/F-ratio change not having had time to affect the engine rotational speed. Further at least one forthcoming revolution of the engine is chosen in such a manner that it is affected by the brief A/F-ratio change. In this manner it becomes possible to compute a revolution-time difference caused by an A/F-ratio change. On the basis of this revolution-time difference a change, if needed, of the mixture ratio in the desired direction towards a leaner or richer mixture is made. Thus using this method an optimal mixture can be achieved by testing how the engine reacts to a leaner or richer mixture.
However, the engine control method of EP 0 715 686 B1 is somewhat slow and it would therefore be advantageous if it could be speeded up. For instance if the present A/F ratio comes far off from the desired A/F ratio it would be an advantage if the starting A/F-ratio could be fast-forwarded to a position closer to the desired A/F-ratio before the control method of EP 0 715 686 B1 steps in.
Further, in the context of the application an engine is said to be idling when the engine is running at zero throttle. If the engine is too lean at start it may operate when it is idling, but failing to accelerate to working speed when the throttle is set to full. This may be a problem for a control method active only when at working speed.
When starting the engine at a given fuel supply setting, the corresponding A/F ratio could be affected by a number of factors. For instance if the engine is used in a smoky and hot environment, e.g. as rescue equipment in a fire, providing a high temperature and a deteriorated air quality. But also factors as outside air-pressure and moisture content as well as engine wear, fuel quality and the condition of the air-filter influences the corresponding A/F ratio. Of course having additional sensors could to some extent compensate for such factors, but more sensors increases costs as well as size and weight of the engine, all preferred to be kept at minimum.
Further, in most engines for a power saw, a power cutter, a lawn mover and similar consumer products, the A/F ratio is manually controlled when the engine is idling, e.g. the electronic control system is only active when the engine is at working speed or above. It would therefore be desirable to have a simple, non-expensive but efficient electronic control method, without the need of adjusting the fuel or air supply manually, when the engine is idling.
The situation when the engine is operated at full throttle and at the same time not subjected to any load (other than inevitable friction within the machine comprising the engine) is in the context of the application denoted as a free speed situation and the engine reaches high engine speeds when free speeding. In the prior art it is known to control the A/F-ratio so that the desired A/F-ratio is defined as the A/F-ratio when the engine has reached its maximum engine speed. However, the engine wear is increased with higher engine speeds and higher speeds may result in damages to the engine and a reduced expected service life of the engine.